1. Field of the Invention
The present invention relates to a molded article of fluorine-containing polymer, especially of polytetrafluoroethylene, with increased water-repellency, and to a jig for holding articles to be washed during washing processes using an aqueous washing medium.
2. Description of the Background Art
Fluorine-containing polymers are used widely in many fields of industry because of their excellent chemical resistance, low surface friction, flame-retardancy, and heat resistance.
Freon (trademark) has been commonly used as a major medium for washing various semi-conductor products. Because of environmental problems, however, the use of Freon in these areas has been replaced by super-purified water in recent years. For this reason, fluorine-containing polymer materials having superior water-repellency, such as polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, and tetrafluoroethylene-hexafluoropropylene copolymer, are used for various tools, jigs, parts, and equipment in semi-conductor manufacturing processes. The contact angle with water for these fluorine-containing polymers with water, indicative of the water-repellency of these materials, is as small as about 110.degree., which is insufficient for easy drainage of water. A drying step is thus required for water removal.
Water-repellency of solid surfaces of polymer materials is known to be greatly affected by surface roughness, provided that the chemical composition and the crystalline structure are the same (R. N. Wenzel, Ind. Eng. Chem., 28, 988 (1936)). Known as methods for increasing water-repellency of polymer surfaces are, for example, a method of dispersing a water-repellent filler in a polymer having perfluoroalkyl group as a matrix (F. Oishi, et al., Hyoumen (Surface), 28, 988 (1990)) and a method of performing eutectoid galvanization of a galvanizing solution in which particles of polytetrafluoroethylene oligomer are dispersed (Japanese Patent Application Laid-open (kokai) 285199/1992).
In these methods, however, the water-repellent filler or particles of polytetrafluoroethylene oligomer may be released from the matrix and give rise to decreased water-repellency.
One of the objects of the present invention is therefore to provide an article molded from fluorine-containing polymer, especially of polytetrafluoroethylene, with increased water-repellency.
As mentioned above, washing processes using an aqueous medium are more popular in semi-conductor and precision mechanical parts industries in the wake of promulgation of the restricted use of Freon. In these washing processes, a number of systems are employed for successively immersing parts to be washed, held by a jig such as a semi-conductor wafer washing carrier, in several vessels containing different types of aqueous washing mediums or water.
Polytetrafluoroethylene is known as a typical material used for molding jigs such as carriers employed in these processes. Since water is much less volatile than Freon, however, removal of the water from these tools or jigs made of polytetrafluoroethylene is insufficient. This causes problems such as the need for time-consuming and complex procedures for removing water attached to the washed parts after removing these parts from the washing vessels, which results in a reduction in productivity; and contamination of the washing media by the attached water in subsequent vessels, which gives rise to the necessity of frequent replacement of the washing media in these vessels. Water tends to remain especially in portions where the washed parts and the jig such as a carrier come in contact. Delayed removal of such remaining water is considered to impair the product quality depending on the applications of the product.
Methods conventionally used for avoiding these problems have been a method of extracting the washed products very slowly from the vessel while heating the washing liquid to vaporize the attached water over the liquid surface, or a method of removing water by a centrifuge. These methods are also time consuming and require complicated procedures and high cost.
It is therefore another object of the present invention to provide a jig made of polytetrafluoroethylene exhibiting greatly improved water-removing properties, carrying minimal water attached thereto, and remarkably improving the productivity, when used for holding parts to be washed in washing processes using an aqueous washing medium.
As a result of extensive studies in order to improve water-repellency of molded articles of fluorine-containing polymer, the present inventors have found that a molded article of fluorine-containing polymer with excellent water-repellency and superior durability can be obtained by treating the surface of such a molded article with fluorine gas after roughening the surface. The present inventors have further found that the water-repellency can be also increased in molded articles of fluorine-containing polymer, especially of polytetrafluoroethylene, when the articles are treated with heat at a temperature between the melting point and the decomposition temperature. Furthermore, the present inventors have found that these molded articles of fluorine-containing polymer, especially of polytetrafluoroethylene, are extremely useful as a jig for holding articles to be washed in washing processes using an aqueous washing medium in variety of fields. These findings have led to the completion of the present invention.